I propose that Ca2 ion transport across the erythrocyte membrane is regulated by means of the interaction of certain membrane-associated enzymes or their products. The enzymes are the Ca2 ion-Mg2 ion-dependent adenosine triphosphatase (ATPase), the phosphatidylinositol kinase and/or the phosphatidylinositol phosphate phosphatase, and the protein kinase. These enzymes are being isolated, purified, and characterized kinetically in order that the applicability of the model can be determined. BIBLIOGRAPHIC REFERENCE: Cunningham, E. and Grant, B.F. (1977) Purification of a Protein Kinase from the Human Erythrocyte Membrane, Fed. Proc. (in press).